Electrical and fiber optic cable is often laid within conduit for protection from the elements as well as from damage due to impact, cuts and abrasion. The conduits may be positioned underground and within buildings or other large structures and may extend over distances as long as a mile or more.
Typically, the interior of the conduit is accessible only at isolated end points. Therefore, to lay cable within a conduit, a lightweight pull tape is first blown though the conduit using compressed air or other known techniques. The pull tape extends from one end of the conduit to the other end. One end of the pull tape is then attached to the cable, and the pull tape is drawn out of the conduit from its other end, thereby drawing the cable through the conduit from one end to the other.
Because the lengths over which the cable is drawn may be long and tortuous, the pull tape and the cable may experience high tensile forces, largely due to the weight of the cable and the friction between the cable and the conduit. While the cable and the pull tape individually are designed to take such forces, attachment of the tape to the cable proves to be the weak link in the system.
To provide a means for attaching pull tape to cable, kellems were developed. A kellem comprises an elongated braided gripping sleeve, preferably made of high strength but flexible filamentary members. One end of the gripping sleeve has an eye that allows the pull tape to be tied to it. The other end has an opening allowing the cable to be received within the gripping sleeve. The gripping sleeve surrounds and grips the cable as described below, and allows the cable to be drawn through the conduit by the pull tape attached to the eye at its opposite end.
The filamentary members comprising the gripping sleeve of the kellem are resiliently biased so that, when the gripping sleeve is in a stress free, unconstrained state, the inner diameter of the gripping sleeve is smaller than the outer diameter of the cable. Because the gripping sleeve is braided, compressing the gripping sleeve lengthwise will expand the diameter radially outwardly, while tensioning the gripping sleeve will cause the diameter to contract radially inwardly. This characteristic is known as the “trellis effect” and is exhibited by braided structures.
Taking advantage of the trellis effect, the gripping sleeve is compressed lengthwise, expanding its inner diameter against the resilient biasing forces to a size that allows the cable to be inserted into the gripping sleeve. Once the cable is within the gripping sleeve, the compression force is removed, and the gripping sleeve contracts radially inwardly due to its resilient biasing and grips the cable. During the draw through the conduit, tensile force is applied to the gripping sleeve, causing it to contract further and grip the cable with greater force. Within limits, the more pulling force that is applied to the gripping sleeve, the greater gripping force will be exerted on the cable due to the trellis effect.
At the completion of the pull, the gripping sleeve is compressed lengthwise, expanding its diameter outwardly to release the gripping force on the cable, and the cable is removed from the gripping sleeve.
A disadvantage associated with the above-described kellem is the practice of covering it with a layer of tape, such as electrical tape. The tape is applied after the cable is inserted in the gripping sleeve to prevent the outer surface of the gripping sleeve from directly contacting the conduit, other cables or protective sleeves within the conduit. The surfaces of the filamentary members making up the kellem gripping sleeve tend to be sharp and rough and, if uncovered, the gripping sleeve can act like a saw, damaging or severing items with the conduit or even the conduit itself during the draw. Furthermore, the open mesh of the braid provides relatively large interstices that can snag items within the conduit.
Typically, large quantities of tape, on the order of about 20 feet or more, are required to fully cover a kellem of even modest length. The taping process is also very laborious in that it takes a lot of time to apply the tape before the pull as well as to remove the tape after the pull is completed. The tape is discarded after a pull.
There is clearly a need for a device that will eliminate the need to tape and untape kellems used to install cable in a conduit.